Interaction chemistry of ammonia and formaldehyde: Multi-species measurements and kinetic modeling

Mitigating greenhouse gas and pollutant emissions ranks among the foremost concerns in our society due to their profound effects on human health and the environment. Using state-of-art multi-species infrared laser absorption techniques, we conducted the first-ever experimental assessment of the chemical interactions between ammonia (NH₃) and formaldehyde (CH₂O) under combustion conditions in a shock tube. The speciation time-histories of NH₃, CH₂O, NO, H₂O, CO, CO₂ and ignition delay times were measured over 1253–1920 K and 1.14–2.36 bar. Our proposed model, featuring an updated NH₃/CH₂O subset, enhances predictability and highlights the intricate chemistry. Nevertheless, the models found in the literature were unable to well capture our measurements. Importantly, we applied high-level ab-initio theoretical calculations to determine the rate coefficients of the crucial reaction NH₂+CH₂O[dbnd]HCO+NH₃. The measurements demonstrate that CH₂O undergoes decay much earlier than NH₃, leading to a substantial temporal domain for the reactions. In the current CH₂O/NH₃ system, we identified three distinct time-dependent reaction domains mediated by HO₂, NH₂ and OH/H radicals. The formaldehyde chemistry (e.g., CH₂O+HO₂ and CH₂O+NH₂) and the amine chemistry (e.g., NH₃+O₂ and NH₃+OH) controls the consumption of CH₂O during the initial stage, ultimately leading to the initial NH₃ decay and heat release in the first stage. Subsequently, the significance of NH₂+HO₂ and HCO decomposition becomes important, leading to the accumulation of heat and NH₂ radical during the CO plateau region. Amine chemistry, including NH₂+NO and NH₂+HO₂, combined with the chain-branching step (O₂+H=O+OH) in H₂-O₂ chemistry, leads to an enhanced generation of H and OH radicals. This enhancement, in turn, promotes the heat release reaction CO+OH[dbnd]CO₂+H, ultimately leading to hot ignition. This study underscores the intricate interplay between formaldehyde and ammonia chemistry, a crucial factor in forecasting the ignition and emission behaviors of hydrocarbon-ammonia systems.